building a reverse rotation merc 260
 

can someone tell me if i can take a standard rotating merc 260 and just remove the timing chain and switch it to timing gears .or are there other mods to be done

Cam, oil pump, water pump all have to be changed I believe.

pistons need to be rotated 180* to correct the thrust offset

notch at the top of the piston that faces the front needs to face the rear

first you need a RR cam to go with that gear /timing chain change. If not your standard cam will open the exhaust first and then the intake valves. The main crank and harmonic balancer seals have to be changed or instead of holding oil in they will leak oil out. If the crank has been used as a standard rotation it will at least need to be polished or honed so the microscopic scales lay the right way with new bearings.
Most RR motors I have dealt with pretty much the only thing that spins backwards is the crank. Since the crank spins backwards you use the timing gears instead on the two sprockets and chain to spin the cam back the normal way. The cam spinning the normal way means the oil pump, distributor and most everything else are all normal off the shelf parts.

i think you will have big problems if you try this.the valve relief will be no where near the valves and contact will happen and destroy the engine.

I pulled a 327 Chevrolet apart that was reverse rotation and that is exactly how they did it, notches to the rear. The pistons had 4 valve reliefs in them though.

Why would you want reverse rotation?

Rear main seal is different from std to reverse rotation. sorry didn't see this was already addressed

Put a Bravo behind a normal rotation. hah !

or a Volvo Penta

big difference from bbc to sbc.a low compression sbc piston has 4 valve reliefs.

IMHO the pistons on the small block still should have went in the right way.

MT - This thread is about a small block.

Why? Wouldn't the offset on the pin be the wrong direction?

No. If the piston is this way or that way the rod stays the same. The piston just slides from one side to the other on the piston pin.
Another concern I would have is if the rod is also on backwards or is it normal on the piston which will be backwards. If the rod is on backwards and if you are using a forged crank with radius corners on the crank journals the narrowed rod bearings will grind on the crank. Bottom line is the bearing tabs go outside always.

The rods were oriented the usual way in the motor I am discussing. The pistons were on opposite of how they normally are so the offset would reduce the thrust load on the reverse rotation.

well crap.i guess i have bbc on the brain,OOPS.

i did not know a sbc has an offset pin in the piston.are you sure about that?

If the Pistons have 4 valve reliefs they can go either way

i have done quite a few that way,i never heard of a sbc with on offset pin bore.

i hope OP wants this engine for an inboard application. you can't spin an alpha or a bravo backwards.

have to use the reverse rotation cam, gear set and should use the rear main seal/ft seals but some don't

thank you guys , i found a reverse rotation sbc that just needed , new rings and bearings ,i found this was easier that trying to change rotation on a standard engine . by the way if you want to know what happened a friend of mine blew a motor n an old wellcraft v-drive and never noticed it was reverse rotation , he put in a standard rotation motor in he called me in because he couldn t start it loll i noticed his error asked him where he put original motor so we could rebuild it . answer scap guy pick up old motor . so now i have a nice merc 260 standard rotation 0 hours in stock he gave it to me and he has a god only knows how many hours reverse rotation motor that i got running for him and he is soooo happy to get his boat back

Just putting a RR SBC into an old Chris Craft, kept all the originals for reference, bought a new RR cam and seals, etc. It has a 71C behind it

http://www.jepistons.com/Images/Tech...pin_offset.jpg


in ref. to post#16 being correct

First off seals: the chain cover and main seal are crank rotation specific. 2nd Camshaft: You'll need to replace the camshaft and tappets with new ones and use special high sulfur lube and remove the dampers on the valve springs during break in There are two methods to turn the cam. It all depends on the cam's distributer helix and lobe configuration. Old school is a 2 gear mesh that spins a standard cam rotation with a reverse firing order. This method is plug and play with the standard distributer gear. Note that only flat tappet camshafts are available unless a custom camshaft is made hence the reason the cam and tappets need to be new. This also means it uses a flat nose cam with the larger bolt circle Dia,. The newer method is to use a chain and sprocket with a reverse cut distributer gear to correct the rotation of the distributer for the oil pump . This method spins the cam reverse rotation with a standard firing order. Generally these are aftermarket cams. The same rules apply to the camshaft as being a flat tappet. There has never been nor will there ever be an off the shelf roller cam for a GM reverse rotation application. Always be very cautious that the camshaft you are using is set up for either the gear set or timing chain set. Also not that Mercruiser/GM made only one flavor of this camshaft. All and every GM RHR small blocks have used the same camshaft in every reverse rotation motor ever made. You can no longer buy the GM camshaft . A stock configuration for the gear set is available through Melling. A cam core is available through Comp Cam and they claim it is a reverse rotation standard firing order camshaft which would have and use the helix for reverse cut distributer gear. I personally had a custom retro fit hydraulic roller camshaft ground from Comp Cams for $850. It uses the gear set with a modified thrust plate and a thrust button in a 5.7 roller block. Lastly I'll clear up the piston orientation. If you use a stock Chevrolet piston it has a .050 wrist pin offset to prevent piston knock when the engine is cold.. It has to be reversed so the notches face the rear of the engine to prevent extreme knock when the engine is cold. Some people will flip the piston on the rods. I chose to reverse the banks so #1 becomes #2 and visa versa and down the line. This corrects the heavy chamfer orientation and also maintains the balance . To each is his own if you feel the need to keep the same piston in the same hole by all means spin the piston around on the rod. Some aftermarket piston do not use an offset so there is no need to change the orientation of the piston. one final note: Some will argue that the Crankshaft is also rotation specific. Maybe back in the hay day when you could get one. Well Today you can't and every modern RHR uses a standard crank. Some will polish the Journals in the opposite rotation. So there it is can you just throw in RHR gear set and camshaft In a LHR? NO you can't